1. Field of the Invention
The invention relates to the field of heated windows, especially those used in aircraft.
2. Description of the Background
Heated windows generally comprise at least one glass sheet coated with one or more transparent electronically conductive films. Conductive strips located on at least two opposite sides of the window allow a current to flow within the film, generating heat by the Joule effect, which heat is useful for de-icing and/or de-misting the window.
Transparent electronically conductive films are generally based on metals, in particular silver, or oxides (called TCOs), especially indium tin oxide (ITO).
Far from being square or rectangular, heated windows, especially those used in aircraft, sometimes have complex shapes; this may especially be a trapezoidal or pentagonal shape. As a result, for electronically conductive films of uniform sheet resistance, i.e. that have a sheet resistance that is substantially identical from one region of the film to another, the electrical power density may become very non-uniform, especially varying by a factor of 4 or 5 depending on the region of the window. The heating intensity is therefore very dependent on the region of the window.
One solution to this problem consists in locally modulating the sheet resistance of the thin electronically conductive film in order to obtain different sheet resistances for different regions of the window. Optimized sheet resistance maps may thus be calculated.
To actually produce such maps, it has been proposed to modulate locally the thickness of the thin electronically conductive film by modifying its deposition conditions. More precisely, when the film is deposited by magnetron sputtering, it has been proposed to place automatically controlled moveable masks between the glass sheet and the cathode. Creating regions with different thicknesses, and therefore with different sheet resistances, allows uniform or substantially uniform electrical power densities to be obtained over the entire area of the heated window.
This solution is, however, not without drawbacks: the deposition time is prolonged, mask maintenance requires the deposition equipment to be stopped and residues deposited on the masks can contaminate the surface of the films, reducing the yield of the method. Furthermore, the thin-film deposition conditions must be finely tailored to each window geometry.